WASHINGTON - The leader of the world's Roman Catholics has been to the White House only once in history. That changes this week, and President Bush is pulling out all the stops: driving out to a suburban military base to meet Pope Benedict XVI's plane, bringing a giant audience to the South Lawn and hosting an East Room dinner.

These are all firsts.

Bush has never before given a visiting leader the honor of picking him up at the airport. In fact, no president has done so at Andrews Air Force Base, the typical landing spot for modern leaders.

A crowd of up to 12,000 is due at the White House on Wednesday morning for the pope's official, pomp-filled arrival ceremony. It will feature the U.S. and Holy See anthems, a 21-gun salute, and the U.S. Army Old Guard Fife and Drum Corps. Both men will make remarks before their Oval Office meeting and a send-off for the popemobile down Pennsylvania Avenue.

The White House crowd will be the largest of Bush's presidency.

The evening festivities will mark the first time the Bushes have put on a high-profile meal in honor of someone who isn't a guest. Wednesday is the pontiff's 81st birthday, and the menu celebrates his German heritage with Bavarian-style food.

But Benedict's prayer service that evening with U.S. bishops at a famed Washington basilica precludes his coming to the dinner, according to the White House. Catholic leaders will be there instead.

The Bush-Benedict get-together will be the 25th meeting between a pope and a sitting president.

There are more than 64 million reasons for this. Catholics number nearly one-quarter of the U.S. population, making them a desirable constituency for politicians. Worldwide, there are now an estimated 1 billion Roman Catholics.

"The pope represents not just the Catholic church but the possibility of moral argument in world affairs, and it is very important for American presidents to rub up against that from time to time," said George Weigel, a Catholic theologian and biographer of Pope John Paul II.

The Vatican has an interest, too.

"It wants to be a player in world affairs, and everyone understands that to do that you have to be in conversation with the United States," said John Allen, the Vatican correspondent for the independent National Catholic Reporter.